In this article, I'd like to take that to the next level and demonstrate how to write an information schema plug-in that can access some of the internals of the MySQL server. For this particular purpose, we will focus on a plug-in that reports all the SAVEPOINTs available in the current session. This MYSQL_SAVEPOINTS plug-in may be of some value when debugging scripts and stored routines that rely on complex scenarios using transactions and savepoints.

In a forthcoming article, I will describe a few information schema plug-ins that are arguably more interesting, such as a plug-in to list the currently existing TEMPORARY tables, user-defined variables, and the contents of the query cache. Although the plug-in described in this article may be of some use, its main purpose is to illustrate the minimal requirements for plug-ins that can access the server's internals.

A Quick Recapitulation

You might recall that:

The MySQL plug-in API is one of the new features in MySQL 5.1, and forms a generic extension point of the MySQL database server, allowing privileged database users to add functionality to the MySQL Server by loading a shared library from the plug-in directory

Loading and unloading a plug-in is a completely dynamic process controlled using the INSTALL PLUGIN and UNINSTALL PLUGIN syntax, and does not involve compiling the server or even restarting it

There are several types of plug-ins, the most well-known being storage engines and full-text parsers. Less well-known types include information schema and daemon plug-ins.

Plug-ins are usually implemented in C/C++. To implement a plug-in, the implementor must include the header file plugin.h and provide an initialized instance of the st_mysql_plugin structure. In addition, the implementor must provide code to implement the plug-in type dependent part of the interface

The plug-in type dependent part of the interface for information schema plug-ins consists of two things: the column definitions of the information schema table and a fill_table function that is called whenever the server wants to retrieve the rows of data from that table.

How Information Schema plug-ins can access MySQL Server internals

Before we discuss the MYSQL_SAVEPOINTS information schema plug-in in detail, let's take a look at the way information schema plug-ins can obtain access to the internals of the MySQL server.

Like I just recapitulated from last week's article, the plug-in type dependent inferface for information schema plug-ins consists of two things:

An array of ST_FIELD_INFO structures, each of which defines a column of the information schema table

A fill_table function that is called by the server when it needs to retrieve the data from table

The column definitions may be considered a static part of the interface - they simply define the structure of the table - no more, no less. The fill_table function is a different matter. Let's take a look at the signature of the signature of that function:

int fill_table(THD *thd, TABLE_LIST *tables, COND *cond);

The TABLE_LIST *tables argument provides the handle to the information schema table that is being filled, and the COND *cond argument represents the WHERE condition of the SQL statement that is currently being handled, allowing the fill_table function to directly filter rows (instead of relying on the query execution engine to do that). As such, these arguments are occupied with the actual delivery of rows of data to the server.

The first argument to the fill_table function offers all kinds of interesting opportunities to see what is going on inside the server. We will discuss it in more detail in the next section.

Public accessors to the current THD instance

The first argument to the fill_table function is THD *thd. This is the so-callled thread descriptor - something that is best thought of as a handle to the current session. Note that in a MySQL context, the terms connection, thread and session are often used interchangeably. However, I find the term thread too broad, and the term connection too narrow. As there are many parts in THD that maintain state regarding the events that occurred since a connection is established, it seems most sensible to think of THD as the server-side implementation of a session.

This session handle or thread descriptor has the form of a pointer to an instance of the THD class. The plugin.h header file contains a forward declaration to this class, but the actual declaration is contained in sql/sql_class.h. The THD class is one of the key data structures in understanding the workings of the MySQL server as it is passed as an argument to many internal server functions. Consequently it provides a wealth of possibilities to create interesting new information schema plug-ins. In fact, the number of possibilities are so great, that a number of common usages has been explicitly set aside in the plugin.h header file.

The plugin.h header file contains a number of function declarations and macros that provide access to the members of a THD instance. I will not discuss all of them here, but highlight just a few just to give you an idea:

thd_test_options() - Find out which options are set. This can be used to find out whether a number of boolean options like big_tables, (general and binary) logging, and autocommit are enabled or disabled.

thd_proc_info() - Should be used by the plug-in implementor before starting a potentially time-consuming operation so the rest of the world can monitor what this session doing. The code set here corresponds to the value reported in the State column by the SHOW PROCESSLIST statement

thd_killed() - Can be used by the plug-in implementor to find out if the thread in which this session lives was killed. If the plug-in is involved in a potentially time-consuming process, the plugin-in implementor should periodically check this and gracefully abort the plugin-ins work when it detects that the thread was killed.

thd_alloc() - Allocates some memory from this session's memory pool. If the plug-in requires some small amount of memory, plug-in implementors should use this rather than the standard malloc() function. Calling thd_alloc(); is likely to be faster because it takes memory out of a pre-allocated pool, reducing contention. In addition, it is more convenient because the memory need not be explicitly freed: it is automatically reclaimed by the pool after handling the current statement.

If you are interested in seeing all these declarations, just open plugin.h and look for comments like this:

plugin.h describes a public interface

In plugin.h, the declarations as described in the previous section together form a public interface to the current session. They are there for the convenience of plug-in implementors and represent a 'safe' way to work with the THD pointer passed to the fill_table function.

To say that these form a public interface is to stay that these are officially supported by MySQL AB. That is: they will be supported officially once the MySQL 5.1 Server is a generally available release. From that point on you can rely on these functions when writing plug-ins in the sense that you do not have to be afraid that they will change. At least, the public interface will remain the same for all forthcoming builds of the 5.1 server. Any interface changes in future releases will involve a proper process, giving everybody the chance to update their code well in time.

Unfortunately, not every function declaration in plugin.h has source code comments. This means that for now, you sometimes need to do some digging in the server's source code to find out what you can do with them. I admit that this situation is not exactly perfect. However, the matter has been reported as a bug, and hopefully, it will be adressed soon.

Beyond the public interface

I just described the public interface plug-in implementors can rely on. A distinct advantage of the public interface is that it takes away a lot of the complexity of the underlying internals of the MySQL Server. However, there will always be cases where the public interface does not offer the features you really need. In those cases, you simply need to be able to work directly on the server internals.

The advantage of directly referencing the server's internals is that you can access all the interesting nuts and bolts and bits and pieces. The downside is that there is absolutely no guarantee that your code will work in another version of the server. The internals are by definition the parts that are not meant to be exposed. As such, it is possible that your code does not work or behaves unexpectedly in another version of the server.

Let's not dwell too long on the disadvantages. Instead, let's focus on the merits of pluggable information schema tables. Granted, it is inconvenient that we may need to make our code resilient to each different build of the server. However, for many applications, it is not very likely that we have to constantly do that.

Even if we do have to change our code, the burden will be on the developer of the plug-in. For each specific build of the server, your code may need to be different. Even if the code itself does not change, you will probably at least have to recompile your plug-in for each specific build of the server. However, your users are still not required to recompile the server itself. They can still install the plug-in without stopping or restarting the server, which in many cases seems more important than bearing the burden of changing the code.

You need to break some eggs to bake an omelet - so if you're hungry, you better get over it and start breaking some eggs ;-)

The MYSQL_SERVER define

In order to access the server's internals beyond the public interface, we need to use some C/C++ preprocessor magic and define MYSQL_SERVER. This define needs to be present before we include any MySQL header (or source) files:

#ifndef MYSQL_SERVER#define MYSQL_SERVER#endif

Throughout the MySQL codebase, there are many sections that are conditionally included or excluded depending on whether MYSQL_SERVER is defined. It is hard to pinpoint the exact effect of adding this definition, because there many spots that use this definition to control conditional compilation.

Normally the MYSQL_SERVER definition need be present only when compiling the server proper, but in this case we need it to let the plug-in code work with internal structures such as THD instances directly, that is, without using the accessors provided by the public interface.

To be absolutely clear: using the MySQL_SERVER define in your code does not mean you must compile your plug-in as part of the server. On the contrary - you can compile your plug-ins separately from the server, and still (un)install them at runtime. The only thing the MySQL_SERVER define does, is pull in the declarations that are normally considered to be 'internal'. They will for example allow us to work directly with the members of the THD class instead of being required to use the public accessors defined in plugin.h.

Implementing the MYSQL_SAVEPOINTS Information Schema plug-in

Now that we sketched the backgrounds, we can quickly proceed and discuss the implementation of the MYSQL_SAVEPOINTS information schema plug-in. (Note that you can download the source code file mysql_is_savepoints.cc here.)

Creating the source file

First, we need to create a C++ source file. We will assume that the working directory is ~/mysql_is_savepoints/, and that the source file is called mysql_is_savepoints.cc.

The MYSQL_SERVER define

Like we explained in the previous sections, we need to define MYSQL_SERVER so we can directly reference the members of the THD class passed to our fill_table function.

#ifndef MYSQL_SERVER#define MYSQL_SERVER#endif

Because this affects how the included files are processed, we do this at the very top of our source file.

Include files

We can use the same list of includes we used for the MYSQL_HELLO plug-in - the MYSQL_SERVER define is responsible for including all the additional things we require to write the MYSQL_SAVEPOINTS plug-ins.

This time, in addition to creating the ST_FIELD_INFO array of column definitions, we also create #defines for the array entry indexes. The defines allow us to refer to the column definitions using the names rather than the raw, literal integer array indexes. This has the advantage that we do not have to change code should we want to change the positions of the columns. Another advantage is that our fill_table code will be easier to read: by consistently referring to COLUMN_SAVEPOINT_ID and COLUMN_SAVEPOINT_NAME rather than 0 and 1 it will be much easier to see what is going on.

Filling the table

Now we come to the heart of the matter: generating a row for each SQL SAVEPOINT available in the current session.

The savepoints for the current session are available in the transaction member of the THD class. The transaction member is an instance of the st_transactions struct, which is declared locally inside the THD class:

class THD :public Statement, public Open_tables_state{

...many, many lines here...

public:

struct st_transactions {SAVEPOINT *savepoints;

...a few more lines here...

} transaction;

...many, many more lines here ...

};

Now you might recall that the THD class is declared in sql/sql_class.h. However, you might have some trouble locating the transaction member, because the declaration of the THD class is extremely large and long-winded: in the MySQL 5.1.22-rc source distribution, it ranges from lines 960 to 1886(!!) - and those 900 something lines make up only the declaration!

(Although the official explanation for the name of the THD class is that it is an acronym for THread Descriptor, some developers* explained that it is one of the few class names that is spelled in capitals because it is so incredibly heavy. According to this anecdote, its name should be pronounced as "...THUD!!...THUD!!..." because of the sound it makes each time it is dumped into the argument list of a function that makes up the servers source code.

Anyway, you will find it easier when you look for st_transactions, or go directly to line 1149, but note that the line number is likely to be different in other versions of the server code.

Now, we can see that the st_transaction struct contains a pointer to a SAVEPOINT pointer called savepoints. As we shall see later, this is actually the list of savepoints we need. But what kind of type is this SAVEPOINT exactly?

Well, to get past this point, you really need some patience and a set of tools that allow you to search the source code. In the case of SAVEPOINT, it turns out that this is actually a typedef for the st_savepoint structure. Now, the odd thing is that this typedef appears in sql/handler.h:

typedef struct st_savepoint SAVEPOINT;

But the structure st_savepoint itself is declared in sql/sql_class.h - that is, the same file that declares THD, which seems to prefer SAVEPOINT rather than st_savepoint!

Well - it is beyond me why it was done like this. For our purpose it doesn't really matter though, let's examine the declaration of st_savepoint instead:

The biggest difference with the fill_table function we used in the MYSQL_HELLO example is that instead of just storing one single row, we have a loop, storing one row for each iteration. The loop is initialized by assigning the SAVEPOINT pointer from the transaction member from the THD instance that is passed as the first argument to the fill_table function to a local sv variable:

SAVEPOINT *sv= thd->transaction.savepoints;

Of course, it is possible that there are no savepoints in the current session, in which case thd->transaction.savepoints will be the NULL pointer. However, if there are savepoints, a pointer to the last savepoint that was created in the current session will now be stored in sv. We can now set up the actual loop:

while(sv && !status) {

...lines here...

status= schema_table_store_record(thd, table); sv= sv->prev; }

Note that the loop will be entered only if sv points to a savepoint. If it does, data from the savepoint is written to the columns of our information schema table.

In the bottom of the loop, we store the current record using the schema_table_store_record, which we discussed already for the MYSQL_HELLO example:

status= schema_table_store_record(thd, table);

Interestingly, we were required to make a forward declaration to it in the MYSQL_HELLO example. Now, we don't have to do this, presumably because we defined MYSQL_SERVER.

You might recall that schema_table_store_record function returns 0 in case of success and 1 instead of failure. Note that if a failure occurs at this point, the loop will not iterate again, as the while condition requires status to be not true (that is, zero).

After storing the row, the last step of the loop is to move back and examine the previous savepoint:

sv= sv->prev;

If the end of the list is reached, sv will be NULL, preventing the loop to iterate again. However, if there is in fact a previous savepoint, the loop will run once again and create a new row for that savepoint too, on and on until we reach the end of the list of savepoints.

In the top of the loop, we store data into the columns of our information schema table:

This time, we use our defines COLUMN_SAVEPOINT_ID and COLUMN_SAVEPOINT_NAME instead of the literal numerical field indexes. We already demonstrated in the MYSQL_HELLO example how to store a string, so we won't discuss the line that stores the savepoint's name. Instead, let's find out how we can store an integer value by looking at the line that stores the savepoint id.

As you can see, we stipulate the value for the SAVEPOINT_ID column ourselves by simply adding one for each row:

table->field[COLUMN_SAVEPOINT_ID]->store(++savepoint_id, 0);

Savepoints by no means have a numerical ID of their own, but it makes sense to make one up in order to unambigously indicate the order in which the savepoints were created during this session. Note the second argument to the store method, which is always 0 here:

table->field[COLUMN_SAVEPOINT_ID]->store(++savepoint_id, 0);

This second argument is there to tell the store method whether the value represents a signed or an unsigned value. In this case, we are storing an unsigned value - it should be 1 for an unsigned value.

The rest of the implementation

The remainder of the implementation is quite similar to what was discussed for the MySQL_HELLO example. The most important difference is actually the plug-in name, but otherwise the implementation is identical. Therefore, it is not discussed here further.

Building and Installing

The build and install process is pretty much similar to that for the MYSQL_HELLO plug-in.

Learn More

This has been quite a ride! In this article it was demonstrated how we can use information schema plug-ins to report some of the things that are going on inside the current session. As such, the MYSQL_SAVEPOINTS plug-in is a big step forward compared to the MYSQL_HELLO plug-in.

However, this is just the tip of the iceberg - the current session harbours much more interesting information about the current session, and in a forthcoming article I will demonstrate a number of other usages. In particular, I will present a plug-in to report the user variables in the current session, and the temporary tables defined in the current session.

In another article, we will also see that it is sometimes possible to look beyond the current session and report on the status of server-wide structures, such as the query cache.

Meet us at the user's conference

When I discussed the MYSQL_HELLO plug-in, I already hinted that there will be a lot there for those people that want to learn more about extending the server with (information schema) plug-ins. You can find all those links in the bottom of that article.

In addition, you can learn a lot about the MySQL Server internals. And...you can learn it from one of the founding fathers: Monty himself will be doing A tour into MySQL's internals. So, I guess that's going to be one of those occasions where you get the opportunity to clear up some of those details in the source code you never quite managed to wrap your head around.

If you register before the 26th of februari, you'll get a $200 discount. There are more discounts available depending on whether you attended before, or if you register together with a number of colleagues; there's special student and non-profit discounts too - check it out here.

See you at the conference! (Bonus points for the first one to ask Monty in the Q&A why SAVEPOINT is typedef-ed in sql/handler.h instead of sql/sql_class.h; double bonus points for the first one to ask Monty if THD is really called like that because it is so heavy ;-)